This application claims the priority of Korean Patent Application No. 2003-36604, filed on Jun. 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to laser desorption/ionization mass spectrometry, and more particularly, to a sample holder for laser desorption/ionization mass spectrometry, which can easily and precisely analyze low molecular weight materials as well as high molecular weight materials.
2. Description of the Related Art
Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has been widely used for analyzing high molecular weight materials, for example, living body substances (such as protein) or polymers. MALDI mass spectrometry is a type of mass spectrometry, in which a sample loaded into a sample holder is desorbed and ionized by applying ultra violet rays or infrared laser beams thereto and the mass of the sample is analyzed by measuring the time of flight (TOF) of ions of the sample.
FIG. 1A is a diagram illustrating a conventional matrix-assisted laser desorption/ionization (MALDI) mass spectrometer, and FIG. 1B is an enlarged view of a portion A of FIG. 1A.
Referring to FIGS. 1A and 1B, in the conventional MALDI mass spectrometer, a metal plate 10, which is generally formed of stainless steel or gold-coated steel, is used as a sample holder for loading a sample 21. A surface of the metal plate 10 is coated with a material layer 20, which is a mixture of the sample 21 and a matrix 22. The metal plate 10 coated with the sample 21 is placed on a supporting means 31 in a vacuum chamber (not shown).
When ultra violet rays or infrared laser beams are applied to the sample 21 and the matrix 22 loaded on the metal plate 10, the sample 21 is desorbed from the metal plate 10 together with the matrix 22 and then ionized. Ions of the sample 21 are accelerated by an electric field formed by grids 32 and then travel to a detector 34 passing through a deflector 33. At this moment, the TOF of the ions can be measured by the detector 34, and the mass of the ions can be measured using the TOF of the ions.
In the above-mentioned conventional MALDI mass spectrometry, the matrix 22, which facilitates the desorption and ionization of the sample 21, is generally formed of organic acid that readily absorbs ultra violet rays. The ionization of the sample 21 has not yet been fully disclosed, but it is known that the matrix 22, which takes up most of the material layer 20, absorbs the energy of the laser beams applied to the material layer 20 and then transfers the absorbed energy to the sample 21 to facilitate the ionization of the sample 21 during or after the desorption of the sample 21 from the metal plate 10.
In the conventional MALDI mass spectrometry, the degree to which the sample 21 is desorbed from the metal plate 10 and is ionized varies depending on the types of the matrix 22 and a solvent used to mix the sample 21 with the matrix 22 because solid structures, which are formed of a mixture of the sample 21 and the matrix 22 on the metal plate 10 after the solvent vaporizes, are not uniform over the metal plate 10. Therefore, the mass spectrum varies depending on how the sample 21 has been prepared. In addition, the mass spectrum of the sample 21 varies from position to position of the metal plate 10. Thus, it is very difficult to obtain consistent measurement results. Moreover, the conventional MALDI mass spectrometry may fail to precisely measure the molecular weight of a polymer because the molecular weight of the polymer may vary depending on the type of a matrix or a solvent.
Furthermore, since the matrix 22 is ionized together with the sample, it may be very difficult to measure or analyze the mass of the sample 21 due to an interference of ions of the matrix 22, especially when the molecular weight of the sample 21 is not much different from that of the matrix 22, in other words, when the sample 21 is formed of a material with a low molecular weight of, for example, 500 Da or lower.
FIG. 2A is a diagram illustrating a mass spectrum of a sample with a low molecular weight, which is obtained by using the conventional MALDI mass spectrometer. Referring to FIG. 2A, many peaks represent masses of matrix ions while few peaks represent masses of sample ions. In FIG. 2A, the mass of a sample is expressed by the atomic weight (m/z) with respect to the electric charge of each of the sample ions.
The conventional MALDI mass spectrometry can be very useful for the analysis of water-soluble proteins but can be inappropriate for the analysis of polymers, in which organic solvents are generally used, because polymers do not have protons or salts, which are necessary for ionization of the polymers. Consequently, there is the need to add salt to the polymer samples.
FIG. 2B is a diagram illustrating a mass spectrum of a polymer, which is obtained by using the conventional MALDI mass spectrometer. Referring to FIG. 2B, there are few peaks representing the masses of the ions of the polymer because salt was not added to the polymer.
In the meantime, if a mixture of the matrix 22 and the sample 21 is a strong acid, it may erode the metal plate 10 so that the surface of the metal plate 10 may become rough such that it would be difficult to desorb the matrix 22 and the sample 21 from the metal plate 10.
In order to solve the above-mentioned problems of the conventional MALDI mass spectrometry, research has been vigorously carried out on alternatives to the conventional MALDI mass spectrometry. One of the alternatives is laser desorption/ionization mass spectrometry using a porous silicon plate, which has been disclosed in U.S. Pat. No. 6,288,390.
In the patented laser desorption/ionization mass spectrometry, a porous silicon plate is used as a sample holder, which means that a matrix is unnecessary. Therefore, it is possible to obtain a mass spectrum of a sample without any possibilities of a matrix and a solvent having undesirable influences on the mass spectrum. In addition, since a matrix is not used, it is easy to interpret the mass spectrum of a sample with a low molecular weight of 500 Da or lower.
However, since a polymer does not contain protons or salts, it is impossible to ionize the polymer for analysis thereof without adding a salt, such as silver trifluoroacetate (AgTFA), to the polymer. In addition, it is rather complicated to manufacture the porous silicon plate.
FIG. 3 illustrates a graphite plate, which is introduced by Hee-jun Kim et al., in Anal. Chem. 72, pp. 5673–5678 (2000). Referring to FIG. 3, a sample 70, which is to be analyzed, is loaded onto a graphite plate 60 without using a matrix. In FIG. 3, white spots represent the sample 70 loaded onto the graphite plate 60. When the graphite plate 60 absorbs ultra violet rays or infrared rays, a portion of the graphite plate 60 is desorbed from the graphite plate 60 together with the sample 70 and then ionizes the sample 70. Therefore, in the case of using the graphite plate 60 as a sample holder, like in the case of using a silicon plate as a sample holder, there is no need to use a matrix, and thus, it is possible to obtain a mass spectrum of the sample 70 without worrying about the influence of the matrix on the mass spectrum. In addition, since the graphite plate 60 includes Na+ ions and K+ ions, it is possible to ionize and measure a polymer, i.e., the sample 70, without adding salt to the polymer.
The graphite plate 60 is manufactured by cutting a graphite rod into pieces with a thickness of about 2–3 mm using silicon oil. The silicon oil, which is a silicon-based compound with a low molecular weight of about 300–400 Da, is easily absorbed into the graphite plate 60 such that it would be very difficult to remove the silicon oil from the graphite plate 60. Therefore, in the case of analyzing a mass spectrum of a low molecular weight material with a similar molecular weight material to that of silicon oil, it can be very difficult to precisely measure the mass spectrum of the low molecular weight material due to the interference of the silicon oil.
FIG. 4 illustrates a mass spectrum obtained from the graphite plate 60. Referring to FIG. 6, many peaks caused by the silicon oil are rather concentrated in a mass range of 300 m/z to 400 m/z. Two peaks in the far left of FIG. 2B represent masses of Na+ ions and K+ ions, respectively.